The present study was undertaken to develop a rapid, quantitative, in vitro system for evaluating antiviral agents, neutralizing antibodies and various cytokine effects on HIV-1 replication. A system was developed to simultaneously test the effects of various agents on the replication of 1) representative strains of HIV, 2) virus presented cell-free or cell-associated, and 3) HIV transmitted to specific target cells, particularly T-cells and monocytes. The test system uses indicator T-cells (H-938) containing stably integrated copies of the HIV-1-LTR-CAT (chloramphenicol acetyl transferase) gene which respond to virus infection with increased CAT activity. For virus inocula, H-9 T-cells were similarly infected with HIV-1 (IIIB, MN, RF, or Z). Infected cells and culture supernatants were cryopreserved for use as cell-associated and cell-free virus inocula. Virus inocula for testing neutralizing antibodies were chosen to produce a 200-fold increase in CAT activity in indicator cells (1.4 X 10^4 per well) cultured for 3 days. In conjunction with the NIAID/WHO Antibody Serologic Project (ASP), the assay was used to evaluate the potency and strain specificity of controls and purified monoclonal antibodies (MAbs, 7 human, 7 mouse) directed toward V3, V2 gp41, and CD4 binding regions of HIV-1 gp120. There were 20 labs participating in the ASP study and the novel "LTR- CAT" target cell assay was among the most sensitive and specific neutralization assays in the study. Results showed that human MAbs tended to be more broadly neutralizing than mouse MAbs, and MAbs directed toward the V3 or gp41 epitopes of HIV-1 tended to be more potent than those directed toward the CD4 binding region of gp120. In addition, neutralization of cell-associated inocula required 1-2 logs higher MAb concentration than cell-free inocula, with one exception. These results have implications for therapeutics under consideration for licensure and vaccine development and were presented at UCLA Symposium on "Prevention of Treatment of AIDS" and are included in an abstract for "Modern Approaches to Vaccines" meeting at Cold Spring Harbor, NY., September 16-20, l992.